Preparing photographic emulsion for transport



Patented Dec. 24, 1946 STES PREPING PHOTOGRAPHIC EMULSION FOR TRANSPORT Thomas Thorne Baker, Pinner, England, assignor to Dacrematt Limited, London, England 1 This invention relates to the preparation of sensitive photographic emulsions which will keep almost indefinitely in normal or tropical climates and can therefore be transported to distant places or kept in stock for long periods before use. It applies particularly to'sensitive emulsions destined to be applied to large areas or surfaces with a spray gun, or to plates, papers or films in places where circumstances make the manufacture of the chemical emulsions themselves undesirable for any reason.

As the result of experiments it has been found that a stabilized silver bromide emulsion can be squeezed into shreds or noodles and said shreds or noodles can be deprived of their water by partial desiccation either under a vacuum or over a strong dehydrating agent. If the shreds are dehydrated so as to have a water content of between six and ten per cent. they can be packed and transported to a distant place in this dry form. When required for use they can be soaked for two or three hours in cold water, and when swollen, remelted by heat, and applied by a spray gun or by other suitable means to a backing or support, especially to large surfaces such as boards or plaques or canvas for aerial reconnaissance maps or for publicity pictures.

A still further improvement can be effected by dehydrating the shreds at an intermediate stage in the preparation of the emulsion. It is well known, for example, that a silver halide such as silver bromide can be emulsified in gelatin, and can then be set to a jelly, and that said jelly after being broken up into shreds or noodles can be washed in sufficient changes of water to rid it of by-products or soluble or deliquescent salts, ammonia, etc. It is also well known that such washed shreds can be remelted and heated or digested at a critical temperature such as 50 C. for a predetermined time, say 45 minutes, during which digestion the emulsion gains very considerably in photographic vigour and speed, that after such digestion the emulsion can be finished by the addition of free soluble bromide, alcohol, chrome alum or formalin, a preservative such as phenol, and a trace of hydrobromic acid or other stabilizer.

In carrying my invention into effect the washed shreds may be remelted and (without giving the critical digestion) the preservative and free silver bromide may be added, and if desired a trace of hydrobromic acid, such as cos. of a /2 solution to 500 grams of washed and remelted shreds. Such a treated but unfinished emulsion is then again set to a jelly, and re-shredded and the 5 Claims.

2 shreds are dehydrated or desiccated. This ma be done by placing the shreds in a beaker or dish in a desiccator over sulphuric acid, or by the application to the containing vessel of a partial vacuum, or by using both these measures.

In this semi-finished state the desiccated shreds will keep for a considerable time, even for years, and when required for use they have only to besoaked in cold water until swollen, then re-melted and cooked at an elevated temperature such as 50 C. for a suitable time.

In this way a complete range of emulsions for various purposes can be prepared in an emulsion laboratory, and can be packed for indefinite storage or export in the minimum of bulk and without fear of bacterial contamination or the formation of chemical fog.

The successful production of a desiccated emulsion has been effected on the following lines: The volume of the reacting halide solution containing weak gelatin has .been reduced to the minimum consistent with avoiding graininess. This is a. matter of comparatively simple test. In the case of gelatin as a vehicle, it is convenient to start with a 6% solution. Other vehicles than gelatin can of course be employed.

The concentration of the silver solution (e. g. plain 'AgNOg in water) must not be unduly increased if graininess is to be avoided and an aqueous solution of one in six has been found suitable.

When the two solutions have been mixed and digested for about 2 hours, a quantity of dry gelatin in sheets, flakes or powder (preferably flakes) equal in weight to about one-and-threequarter times that of the silver halide (less the amount of gelatin used for emulsifying) is stirred in and a further digestion of about 2 hours is given, the digesting temperature being conveniently 52 C. The emulsion is at this stage about two-fifths of the bulk which the finished emulsion would ordinarily be after the usual processes of washing etc.

Chrome alum as a 5% solution is then added in an amount about 1% of the weight of silver halide, as it is impossible to incorporate it at a later stage. The emulsion is then set to a jelly.

The jelly is next shredded (i. e. squeezed through a perforated plate) in extra cold water to prevent unnecessary swelling, sayat 7-10 C. and the noodles or vermiform shreds are washed in six or eight changes of cold water until the pH of the wash water is about 8. The shreds (still not adequately washed to remove all byproducts) are drained preferably for 1 or 2 hours by standing on a sieve or muslin drum.

The drained shreds are now put through the shredding machine a second time using an innocuous gauze say of nickel or silver surface having a mesh of about 30 per inch, and the shreds are thus broken up into comparatively small particles and are dropped into methyl or ethyl alcohol or acetone or other suitable liquid which is greedy for Water, and the particles are allowed to stand therein for some hours, preferably 24 hours. This treatment has two effects: (1) it toughens the particles so that in the subsequent drying process they do not stick together and form lumps, and (2) it extracts about onethird or a half of the water from the gel.

The partially-desiccated particles are now laid on trays covered with an innocuous gauze, e. g. having a nickel or silver surface, and are dried by gentle heat. Preferably the heating starts at about 21 C. and gradually rises to 32-'35 C. as drying proceeds. The particles are frequently turned over and a revolving gauze, cage or drum can be employed.

Preferably the emulsion should be of such a character that no free bromide or other stabilizing agent is required in the finished product. In the event of such an addition being necessary, however, it may be added either to the spirit or like dehydrating agent (in a little water if necessary) or it may be applied by a spray to the shreds or particles as they lie on the drying trays.

The final shreds, noodles or granules represent in weight about one tenth of the weight that the emulsion would have had if finished in the ordinary way. Thus 5 litres of emulsion would in the dry form weigh approximately 500 grams. This quantity, when soaked in the necessary amount of water to make 5 litres, and then heated up to about 40 C. with stirring, gives an emulsion having the same qualities as that of the emulsion which would be predicted from the same basic formula. Preservatives can be used, such as thymol or phenol or sodium salicylate, which can be conveniently dissolved in the dehydrating spirit or liquid,

Another practical example of this invention is as follows.

Three solutions are prepared:

I. Water, say 12,000 cc.; ammonium bromide, 650 grams; potassium iodide, 1 gram; soft gelatin, 700 grams.

II. Water, 400 00.; silver nitrate, 100 grams; 0.880 ammonia sufficient to redissolve the silver oxide at first precipitated.

III. Water, 3,500 ccs., silver nitrate, 900

grams.

These solutions are made at about 40 C.

Solution II is added to Solution I. Then Solution III is added. Three kilos of dry gelatin is then added and dissolved in the emulsion, which is then set in ice. It is then shredded and washed vuntil the pH is 8, i. e. about ten changes of cold perature gradient beginning at 21 C. and rising to 32 C. or thereabouts. The shreds are stirred, preferably mechanically, during drying.

If desired, a small quantity of a bromide soluble in spirit such as sodium or ammonium bromide can be added to the dehydrating spirit, though it is preferable to make so clean an emulsion that it can eventually be developed for the maximum time without bromide without fogging. In the case of colour sensitive emulsions, the se sitizing dye, e. g., erythrosin, can be added to the spirit. Where it is desired to make desiccated emulsion that withstands warm processing conditions, as much as grams of chrome alum can be added in solution to the above quantity of emulsion prior to washing.

I claim:

1. The process of preparing dry particles of a photographic silver halide emulsion which comprises preparing a silver halide emulsion, setting said emulsion, shredding the set emulsion, washing the shredded emulsion,-reshredding the emulsion into fine particles, soaking said particles in an organic dehydrating liquid, removing said liquid, and drying aid particles.

2. The process of preparing dry particles of a photographic gelatino-silver halide emulsion which comprises preparing a gelatino-silver halide emulsion, setting said emulsion, shredding the 3. The process of preparing dry particles of a photographic gelatino-silver halide emulsion which comprises preparing a gelatino-silver halide emulsion, setting said emulsion, shredding the set emulsion, Washing the shredded emulsion, reshredding the emulsion into fine particles, soaking said particles in a volatile organic dehydrating liquid, removing said liquid, and drying said particles until the water content of the emulsion is not more than 10%.

l. The process of preparing dry particles of a photographic gelatino-silver halide emulsion which comprises preparing a gelatino-silver halide emulsion, setting said emulsion, shredding the set emulsion by extruding it through a perforated plate, washing the shredded emulsion, reshredding said emulsion by extruding it through a fine screen having a mesh of to 40 per square inch, soaking the resulting fine particles in a dehydrating liquid taken from the group consisting of. methyl alcohol, ethyl alcohol and acetone, draining off said liquid and air-drying said particles at a temperature of 21 to C, until the Water content has been reduced to 6 to 10%.

5. The process of preparing dry particles of a photographic gelatino-silver halide emulsion which comprises preparing a gelatino-silver halide emulsion, setting said emulsion, shredding the setemulsion by extruding it through a perforated plate, remelting and digestin said emulsion, setting the digested emulsion, washing the shredded emulsion, reshredding said emulsion by extruding it through a fine screen having a mesh of 30 to per square inch, soaking the resulting fine particles in a dehydrating liquid taken from the group consisting of methyl alcohol, ethyl alcohol, and acetone, draining oil said liquid and air-drying said particles at a temperature of 21 to 35 C. until the water content has been reduced to 6 to THOMAS THORNE BAKER. 

